Colombia's ELN rebels have admitted to killing an indigenous leader last week, drawing criticism from the government and complicating a ceasefire and negotiations to end more than 50 years of war.

The National Liberation Army (ELN) and the government began their first ever bilateral ceasefire at the start of this month, part of peace talks taking place in Ecuador. The ceasefire is set to run through Jan. 9 and may be extended.

The ELN said in a statement released late on Sunday that one of its units in restive Choco province shot Aulio Isarama after detaining him for questioning about his alleged relationship to military intelligence.

The indigenous leader refused to walk to the place where the unit planned to interrogate him, the statement said, and he rushed at one of the rebels “with tragic results.”

Reuters could not independently verify the events described in the statement.

“At no time had an order or authorization been given to attack the physical integrity of the leader Aulio Isarama. The only purpose of the guerrilla unit which detained him was to investigate,” the statement said.

The rebel group added it was committed to recognizing its error and telling the truth about the incident.

“We reiterate to Colombia our commitment to and compliance with the temporary bilateral ceasefire and we will maintain it until it ends,” the rebel group said.

The government’s peace commission, which is leading talks with the rebels, issued a statement condemning the killing and said it hoped the group would allow the investigation into Isarama’s death to proceed quickly, and for those responsible to be brought to justice.

The events will be reviewed in relation to the continuation of the ceasefire, the peace commission's statement added.

“No single event by itself will cause the rupture of the ceasefire in a unilateral and automatic way. The negotiating teams will evaluate all the objective information that they receive to make decisions regarding the ceasefire.”

The rebels will have to take action to ensure incidents like the death of Isarama do not occur again, the peace commission added.

The insurgent group, founded in 1964, had pledged to suspend hostage-taking, attacks on roads and oil installations, the use of landmines and the recruitment of minors during the ceasefire.

In turn, the government agreed to improve protection for community leaders and conditions for about 450 jailed rebels.